Differential GPS

Differential GPS (DGPS) is widely used to improve upon GPS accuracies. Here corrections to the pseudoranges (and / or carrier phases) are computed to improve the positional accuracy of a user’s GPS receiver. DGPS corrections can be applied in either post processing or real-time. DGPS can generally improve positional accuracy to between a metre and a centimetre depending on the signals used, user equipment and methodologies adopted.

Types of DGPS

Ground Based

You can set up your own surveyed-in base station (often used on large construction sites). This sends correction data to the “rover” receivers on site.

You can subscribe to a number of virtual reference station (VRS) systems available in most countries nowadays. These are a dense system of surveyed ground stations which cover national areas and provide corrections via UHF radio or by cellular communications. The Ordnance Survey in UK runs the national OS Net GNSS infrastructure of 110 base stations. OS Net provides free GPS and GLONASS data products as well as a commercial (fee based) high accuracy DGPS service and RTK service.

Satellite Based (SBAS)

There are free (government based) SBAS (Satellite Based Augmentation Systems), which use wide area DGPS corrections from ground stations which are uploaded to geo-stationary satellites (above the equator) which then transmit corrections back down to the roving receivers on the ground. (SBAS is called EGNOS in Europe and WAAS in USA).

WAAS was originally developed as an air navigation aid by the Federal Aviation Administration to improve GPS accuracy, integrity, and availability. WAAS uses a network of ground-based reference stations to monitor the GPS satellites signals, and geostationary satellites to transmit information to users.

The European Geostationary Navigation Overlay Service (EGNOS) is a satellite based augmentation system developed by the European Space Agency, the European Commission and EUROCONTROL. It is intended to supplement GPS, and potentially GLONASS and Galileo by providing integrity messages, corrections and additional ranging signals.

There are also subscription based commercial SBAS systems such as Veripos and Omnistar.

Coastal

The UK General Lighthouse Authorities’ DGPS service is a network of 14 ground based reference stations providing corrections to GPS via MF radio transmissions out to at least 50 nautical miles from the coast around the United Kingdom and the Republic of Ireland.

Using SBAS can improve L1 code base horizontal fix accuracy from 15m to 1 or 2m, using a Commercial Grade GPS receiver.

Using a high quality ground based DGPS and a high quality Professional Grade GPS receiver could improve L1 code based horizontal fix accuracy to sub half metre (30cm to 50cm). Post processing of data from this same system could potentially deliver a horizontal fix accuracy of 10 to 20cm. (However post processing requires greater dwell time on ground points, and is slower in terms of usage and doesn’t deliver results in real-time and so is not frequently used).

If you need better accuracy than the 30cm to 50cm indicated above, then you need to use a carrier based range measurement system called RTK (Real Time Kinematic).

The C/A code signal is clocked at approximately 1MHz, whereas its L1 carrier is clocked at 1575 MHz. This gives wavelengths of 300m and 190cm respectively, for code and carrier.

At 1% of wavelength measurement discrimination, this gives a potential measurement accuracy of 3m and 1.9cm respectively for code and carrier so carrier based RTK is much more accurate. The problem is it is much harder to resolve and advanced mathematics are needed to resolve the so-called ambiguity problem with RTK. This increases the cost of the receiver electronics.

Whilst it is possible to carry out carrier based (RTK) measurement on the L1 carrier alone, in practise it is very slow and somewhat unreliable. As a result, professional users seeking the very highest GPS accuracy, and optimum reliability and speed of use with real-time output of position, use a system of measurement called dual frequency RTK (e.g. carrier phase measurement using both the L1 and L2 carriers) which can give accuracies of 1 to 2cm in plan position. Using 2 carriers cancels out errors due to ionospheric effects.

Cost of receiver electronics increases almost exponentially with accuracy requirement.

It should be noted that vertical fix accuracy is typically 2 to 3 times worse than horizontal (plan) fix accuracy.

Blackroc manufacture a range of professional grade GNSS solutions which enables satellite fix accuracy in real time from submetre to 1cm (RTK).

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